Radio frequency (RF) power amplifiers (PAs) are used in the amplification of RF signals for transmission from a wireless-enabled device. One measure of the quality of an RF PA is the ability of the device to accurately replicate the amplitude of an RF input signal in proportion to a gain of the RF PA. Known as the linearity of the RF PA, this measure determines the amount of amplitude modulation (AM) distortion produced by the device. An additional measure of distortion, known as phase modulation (PM) distortion measures the ability of the RF PA to accurately reproduce the phase of an RF input signal. In an effort to increase the linearity of an RF PA and ensure accurate phase reproduction of an RF input signal provided thereto, conventional solutions have used one or more feedback loops surrounding the device. While generally effective at reducing distortion and thus increasing linearity of a target device, distortion correction or linearization feedback loops often result in increased noise in the RF signal path of the RF PA due to up-conversion and down-conversion of noise. Although there are many well-known noise reduction techniques currently available to designers, they are generally at odds with the operating principles of the distortion correction or linearization feedback loops. That is, while the distortion correction or linearization feedback loops generally require high loop gain and bandwidth to properly function, noise reduction techniques require a low loop gain and bandwidth. Accordingly, the development of low noise RF PAs including distortion correction or linearization feedback loops have been slow to develop, if at all.
FIG. 1 shows a conventional RF PA 10 according to one embodiment of the present disclosure. The conventional RF PA 10 includes an RF input node 12, an RF output node 14, a driver stage amplifier 16, and an output stage amplifier 18. As further shown in FIG. 1, a first noise source N1 is located before the driver stage amplifier 16, while a second noise source N2 is located before the output stage amplifier 18. As will be appreciated by those of ordinary skill in the art, the noise sources N1 and N2 are not actual components but rather visual representations of noise present in the RF signal path between the RF input node 12 and the RF output node 14.
The operation of the conventional RF PA 10 is now discussed with respect to FIGS. 2A through 2C. As discussed above, it is desirable for an RF PA to have a linear gain response over a desired input power range. Absent such a linear response, distortion compensation techniques such as feedback loops must be employed. As shown in FIG. 2A, the driver stage amplifier 16 has a relatively linear gain response over a desired input power range (up to P-MAX). Further, as shown in FIG. 2B, the output stage amplifier 18 also has a relatively linear gain response in the desired input power range. Accordingly, external distortion compensation techniques such as feedback loops may not be necessary for the conventional RF PA 10. FIG. 2C shows a plot of the overall noise for the conventional RF PA 10. Due to the linear gain response thereof, the noise in the conventional RF PA 10 has a relatively flat response that is easily compensated for. Unfortunately, the gain response of the driver stage amplifier 16 and the output stage amplifier 18 shown in FIGS. 2A and 2B, respectively, are generally not achievable in real-world scenarios. Specifically, to achieve these linear gain responses, a large amount of power must be used in the biasing of the driver stage amplifier 16 and the output stage amplifier 18, thereby degrading the efficiency of the conventional RF PA 10. Further, other non-ideal operating characteristics may skew the gain response of either the driver stage amplifier 16 or the output stage amplifier 18, thereby requiring external intervention.
FIG. 3 shows the conventional RF PA 10 including distortion compensation circuitry 20. As shown in FIG. 3, the distortion compensation circuitry 20 is coupled in a feedback and/or feedforward configuration such that a signal at both the RF input node 12 and the RF output node 14 are delivered to the circuitry. Further, the distortion compensation circuitry 20 is coupled to the driver stage amplifier 16, which is a variable gain amplifier, and a phase shifter 22 coupled to the RF signal path. A third noise source N3 is shown between the distortion compensation circuitry 20 and the driver stage amplifier 16, and a fourth noise source N4 is shown between the distortion compensation circuitry 20 and the phase shifter 22.
The operation of the conventional RF PA 10 shown in FIG. 3 is now discussed with respect to FIGS. 4A through 4C. As shown in FIG. 4B, the gain response of the output stage amplifier 18 is relatively flat up to a certain input power (labeled PNL, where NL represents non-linear). At this point, the gain of the output stage amplifier 18 rapidly expands and then contracts. This gain response is much more realistic than that shown in FIG. 2B, as it reflects a non-ideal operating scenario that is often experienced in the real world. In an effort to compensate the gain response of the output stage amplifier 18, the gain response of the driver stage amplifier 16 is adjusted by the distortion compensation circuitry 20 such that the gain response of the driver stage amplifier 16 is equal but opposite to that of the output stage amplifier 18 during the portion of non-linearity between PNL and PMAX, as shown in FIG. 4A. Accordingly, the non-linear portions of the gain response of each one of the driver stage amplifier 16 and the output stage amplifier 18 cancel, resulting in an overall gain response of the conventional RF PA 10 that is linear. Although FIGS. 4A and 4B are discussed with respect to the gain of the driver stage amplifier 16 and the output stage amplifier 18, these graphs could also represent non-linear phase responses of the respective amplifiers, in which case the distortion compensation circuitry 20 would adjust a desired phase shift of the phase shifter 22 in lieu of the gain response of the driver stage amplifier 16.
While generally effective at normalizing the overall gain or phase response of the conventional RF PA 10, the non-linear gain (or phase) response of each one of the driver stage amplifier 16 and the output stage amplifier 18 results in significant up-conversion and down-conversion of noise present in the conventional RF PA 10 (as represented by noise sources N1 through N4). Accordingly, the overall noise response of the conventional RF PA 10 is shown in FIG. 4C, and includes significant peaking between PNL and PMAX.
The reason for the increase in noise due to non-linearities in the driver stage amplifier 16 and the output stage amplifier 18 is now discussed with respect to FIG. 5. As shown in FIG. 5, some level of noise is inherently present in the conventional RF PA 10. Specifically, the noise response of the conventional RF PA 10 has relatively high amplitude at low frequencies, which quickly declines as the frequency increases. Due to non-linearities in the gain response of the driver stage amplifier 16 and the output stage amplifier 18, noise at various multiples of the RF frequency of interest (fRF—the frequency of the RF signal being amplified by the driver stage amplifier 16 or the output stage amplifier 18) plus or minus a certain frequency offset (Δf) is either up-converted or down-converted to the RF frequency of interest fRF plus the frequency offset Δf, thereby resulting in a noise peak. For example, noise present at harmonics of the RF frequency of interest fRF+Δf such as (2n)fRF+Δf and (2n+1)fRF+Δf is down-converted, while noise at one or more lower frequencies is up-converted, resulting in the noise peak shown at the RF frequency of interest fRF+Δf. The noise peaking discussed above can lead to failure of the conventional RF PA 10 to meet various operational requirements, for example, mandated by one or more wireless communications standards. On example of this is the noise generated by a PA transmitter in a receive band (where fTx=fRF and fRX=fRF+Δf.
Accordingly, there is a present need for low noise and high linearity RF PAs including RF PAs with one or more distortion correction or linearization feedback loops.